There already IS a root user (with no password) in every MySQL installation by default. Think about if for a minute and you will see why.

XAMPP/LAMPP (once installed) is 'ready to work'

I recommend you NOT try to use the MySQL CLI (unless you understand Linux pretty well and can grok the differences between XAMPP and a standard LAMP stack). Things are in 'different places' and have 'different owners'. So 'standard' Linux/LAMP commands may get ambiguous results. That is the source of your issue. That mysql command did not know where to find MySQL, that's why it wanted to install the client.

the mysql executable is in /opt/lampp/bin. SOOOO, just like LAMPP (/opt/lampp/lampp ...) SO you should start the MySQL monitor with commands like so: